Accessory Dwelling Unit (ADU) Permit in Longmont, Colorado

Longmont requires an accessory dwelling unit (adu) permit for: Applicant must provide proof of Longmont residency: valid Colorado driver's license or state ID, plus at least two of: vehicle registration, voter registration, tax return/financial documentation, or a utility bill; Applicant must own a detached home in Longmont that was legally permitted at time of construction; Allowed only in zone districts permitting ADUs: Rural Residential (R-RU), Residential-Single Family (R-SF), Residential-Mixed Neighborhood (R-MN), Residential-Multifamily (R-MF), Mixed Use-Downtown (MU-D), Nonresidential-Agricultural (N-AG) — per Table 15.04.020 of the Land Development Code; HOA covenants cannot prohibit ADUs (now unenforceable); HOA plan review/approval is optional but recommended and should be submitted with the permit application if obtained; One ADU allowed per lot; up to 800 sq ft regardless of principal dwelling size, OR up to 50% of the principal dwelling's finished above-ground floor area if larger than 800 sq ft; Total floor area of all accessory structures (including the ADU) capped at 75% of the principal structure's floor area, with specific exemptions for attached/internal ADUs; Detached ADU height cannot exceed the height of the main house; 3-foot all-weather (concrete or similar) sidewalk connection required linking the ADU entrance to the adjacent public sidewalk; wayfinding signage required if the ADU entrance isn't visible from the public right-of-way; Setbacks (new detached ADU): front — not closer than the principal dwelling; rear — 5 ft; side — 5 ft; ADUs may NOT be sold separately from the principal dwelling, and may NOT be leased/rented for tenancies of less than 30 days (no short-term rentals); Mobile homes, RVs, and travel trailers cannot be used as an ADU; St. Vrain Valley School District fees must be documented as paid (contact Ryan Kragerud, Kragerud_ryan@svvsd.org, 303-682-7317). If your project isn't listed, confirm with City of Longmont — Building Services (Building Inspection Division), Development Services Center at (303) 651-8332 before starting work.

In Longmont, the published Total City Fees (typical range, all-inclusive) is: $5,000–$10,000. Additional published fees: Building Permit Fee — valuation-based (same Primary Fee Schedule as new construction) — Varies by valuation — see Residential Building Permit fee tiers. These are the jurisdiction's own published figures — see the fee schedule table and source citations on this page.

Plan review for an accessory dwelling unit (adu) permit in Longmont typically runs 0–0 business days. This is an estimate, not a published figure — confirm current review times with City of Longmont — Building Services (Building Inspection Division), Development Services Center at (303) 651-8332.

You'll need: Accessory Dwelling Unit Application (via ACA portal); Development Services Center Accessory Dwelling Unit Guide (Updated December 2025); Engineered, Stamped Foundation Plan and Shear Wall Design; Survey / Improvement Location Certificate (ILC); Site Sketch; Energy Compliance Path / Manual J, Manual D, Manual S; Soils Report. Depending on your project, Longmont may also ask for: Asbestos Testing Report. See the required-documents checklist on this page for details on each.

Apply through City of Longmont — Building Services (Building Inspection Division), Development Services Center at 385 Kimbark St., Longmont, CO 80501. Phone: (303) 651-8332, email: building.inspection@longmontcolorado.gov. Office hours: Mon–Fri, 8 a.m.–5 p.m. (closed Federal holidays, Veterans Day, and the day after Thanksgiving; Building Inspection closed 10–11 a.m. on the 2nd and 4th Thursday for staff training). Official information: https://longmontcolorado.gov/building-inspection/permits/.

No. Per the City of Longmont's ADU Guide, accessory dwelling units may not be leased or rented for tenancies of less than 30 days — short-term rentals are not allowed.

An ADU may be up to 800 square feet regardless of the principal dwelling's size, or larger if it does not exceed 50% of the finished, above-ground floor area of the principal dwelling. If located in an existing basement, this size restriction does not apply. Total accessory structure floor area (including the ADU) is capped at 75% of the principal structure's floor area, with some exemptions for attached/internal ADUs.

Yes. During the ADU Plan Check phase, Longmont Power & Communications (LPC) reviews submitted plans alongside Public Works, Building, Planning, and Emergency Services to ensure compliance with all applicable codes and utility infrastructure requirements.